This invention relates generally to tank bioleaching of a base metal.
In tank bioleaching of a sulphide concentrate which contains a metal such as copper, nickel, cobalt or zinc, the particular compounds are rendered soluable and are therefore recoverable from solution by making use of an appropriate technique. Some metal compounds, however, which are associated with the concentrate, are not rendered soluble during the bioleaching process. Typically these are compounds of metals such as silver, gold, platinum and palladium which are found in a non-soluble particulate fraction of a bioleaching residue, either complexed among mineral precipitates or in unoxidized original mineral concentrate fractions.
Under the bioleaching conditions these compounds are either completely insoluble (e.g. gold) or are transiently soluble (e.g. silver). While silver is momentarily dissolved under bioleaching conditions (i.e. is transiently soluble), it is rapidly removed from solution by chemical precipitation reactions which typically result in argentojarosite, AgFe3(SO4)2(OH)6. Equilibrium soluble silver concentrations in typical high temperature bioleaching solutions, for sulphide mineral concentrations that contain small quantities of silver, are usually less than 3 mg/t.
FIG. 1 indicates that silver in a transient soluble phase 12, before being removed from solution by precipitation 52, interacts particularly with archaea cells 50 used in a high temperature bioleaching process. Such interaction involves the rapid transfer of silver across the cell membrane where the silver most likely interacts, with high affinity, with compounds contained in the cell cytoplasm and eventually results in the formation of silver nodules inside the cell. FIG. 2 shows that this phenomenon is clearly observable by transmission electron microscopy. Cellular constituent compounds with a high affinity for silver may be sulphur-containing amino acids or the imidizole group of histidine, amongst others. The result of such cross-membrane transfer and internal cell accumulation of silver is that the microbial activity and growth are severely affected, to the extent that these factors may render a bioleaching process inoperable.
Transiently-soluble silver is particularly damaging to bioleaching archaea (used for high temperature bioleaching at 60° C- 80°C), while bacteria used in low temperature bioleaching at 30°C- 45°C are relatively unaffected. The difference in sensitivity to silver in this context is mainly ascribed to the known differences in bacteria and archaea cell membrane structures, chemistry and configuration.
In some instances, due to the specific solution chemistry, operating conditions, or microbial population utilized, or a combination of these factors, the silver does not unduly accumulate in the microbial cells and an active bioleaching process of a silver-containing sulphide mineral concentration can be achieved. The result of such a process, however, is that the silver reports to the residue, mainly as argentojarosite. Recovery of the silver from this residue is expensive and requires a significant amount of processing, either by smelting or by hydrometallurgical processing.
Base metals, such as copper, are generally not inhibitory to archaea cells, but some form of inhibition is encountered at concentrations above 10 g/l. The operation of bioleaching tanks where the copper concentration is about 30 g/l. causes a reduction in microbial activity. It is thus desirable to place a ceiling on the metal-concentrate processing rate in order to avoid soluble copper concentrations levels which exhibit severe inhibitory effects. A benefit would be that the ceiling would prevent solution copper concentrations from reaching inhibition levels and thus allow for increased throughput and processing rates of copper concentrates.